Speciality beverages in which at least a portion is made up of frothed or heated milk are becoming more and more popular. The best-known beverage of this type is a coffee of the cappuccino type. It comprises a liquid portion consisting of coffee topped by a layer of frothed milk which, because of its very much lower density, floats atop the surface of the liquid.
Various systems are known to condition fresh milk. Such prior art systems are for example disclosed in DE 11 31 372, DE 15 54 670, DE 196 11 450, DE 196 24 648, DE 197 19 784, DE 40 37 366, DE 44 45 436, DE 89 15 094, DE 20 2007 009 537, DE 10 2004 063 285, DE 10 2006 031 064, EP 0 344 859, EP 0 480 928, EP 0 607 759, EP 0 791 321, EP 0 813 834, EP 0 858 757, EP 1 197 175, EP 1 223 839, EP 1 501 398, EP 1 597 992, EP 1 716 796, EP 1 725 124, EP 1 731 065, EP 1 776 904, EP 1 776 905, EP 1 785 074, EP 1 827 188, EP 1 857 028, EP 1 878 370, FR 769 848, FR 2 708 185, U.S. Pat. No. 2,932,493, U.S. Pat. No. 3,182,975, U.S. Pat. No. 3,356,349, U.S. Pat. No. 4,162,855, U.S. Pat. No. 4,537,332, U.S. Pat. No. 4,479,908, U.S. Pat. No. 5,133,247, U.S. Pat. No. 5,265,519, U.S. Pat. No. 5,295,431, U.S. Pat. No. 5,473,972, U.S. Pat. No. 6,006,654, U.S. Pat. No. 6,318,247, U.S. Pat. No. 6,712,497, U.S. Pat. No. 7,021,206, U.S. Pat. No. 7,322,282, US2002/0134248, WO 90/10411, WO 96/22830, WO 03/003888, WO 2004/043213, WO 2006/050900, WO 2006/122916, WO 2007/095770, WO 2007/120038, WO 2007/144735, WO 2008/034708, WO 2008/046837 and PCT/EP08/056349. Conditioning systems for other applications, such as tea, coffee, hot chocolate, soup, salad dressing, etc. . . . are disclosed in US 2001/0036124, US 2006/0018187 and U.S. Pat. No. 7,314,307.
The simplest method to froth milk consists in plunging a steam supply pipe into a cup of milk to be frothed. However, the results of such a method depend largely on the skills of the user and generally provide a poor froth quality. In fact, usually, milk ends up merely heated and not significantly frothed for lack of air incorporation into the milk during the conditioning treatment. Indeed, for achieving an acceptable frothing, the user is expected to skillfully move the steaming pipe in the milk so as to agitate it at the same time for simultaneous incorporation of air into the milk. Such systems are for instance disclosed in EP 0 791 321 and EP 1 716 796. The mere heating by vapour without frothing of milk is well known and may also be achieved automatically, for instance as disclosed in EP 1 776 904. In particular, a vapour source may be combined with a stirrer to avoid burning of milk on the steam injection duct by injecting the vapour into the milk via the stirrer enclosed in a stirring chamber in fluid communication with milk, as for instance disclosed in U.S. Pat. No. 3,182,975 and U.S. Pat. No. 4,479,908.
Milk may also be frothed by sucking fresh milk from a milk source into a device that includes an air inlet and a pressurised vapour inlet forming a Venturi arrangement, for heating and mixing such milk with vapour and air, and an outlet for the thereby frothed milk. Such devices are for example disclosed in the abovementioned EP 1 716 796, DE 10 2006 031 064 and DE 44 45 436. Such systems may also be combined with an in-line stirrer, as for example disclosed in DE 20 2007 009 537. Furthermore, in EP 1 785 074 milk is pumped from a reservoir into a mixing chamber into which a mixture or vapour and milk is injected for frothing the milk, and then the frothed milk is delivered to a delivery opening.
Another known approach involves the use of a frothing receptacle which comprises a frothing arrangement and into which milk is supplied and then frothed. The frothing may be carried out by injecting vapour and air with a Venturi arrangement into the milk contained in the receptacle, for instance as disclosed in EP 1 725 124, WO 01/26520 and WO 2006/122916. This system produces frothed milk with a relatively liquid foam having large bubbles.
A problem involved with the suction of milk in a Venturi-type vapour arrangement is the immediate condensation of vapour in the Venturi arrangement and the poor formation of vapour bubbles in the milk.
Frothing of milk in a tank can also be achieved by an impeller for mechanically agitating and driving air into the milk and a resistive heater for heating the milk at the same time, as for example disclosed in WO 2006/050900 and PCT/EP08/056349. This system produces a relatively dry firm foam with large bubbles as well.
There are frothing devices that combine a vapour supply pipe for heating and a mechanical stirrer extending therebelow for simultaneously agitating the milk in a cup held by a used under this frothing device. Such a stirrer may be connected via a drive shaft to a drive rotor that is located inside the vapour tube and that is driven by the passage of vapour in the tube. Vapour, or a mixture of vapour and air, is delivered vertically above the stirrer. For example EP 1 197 175 discloses a vapour pipe having a plurality of openings for incoming milk and air and at its outlet a stirrer for agitating the milk. DE 10 2004 063 285 discloses a similar device in which the vapour pipe is arranged for guiding only vapour and is impervious so as to avoid entry of milk into the tube for hygiene reasons. EP 1 597 992 discloses another such device of the same type. In WO 2007/095770 a lance with a Venturi arrangement is arranged to inject a steam-air mixture in a receptacle of milk. A similar arrangement is disclosed in WO 2007/144735.
A problem with such systems comes from the fact that the user has to properly hold the cup under the vapour pipe, or vice versa, so that the pipe's outlet does not emerge from the milk during the treatment which would lead to the projection of milk all over the place. This risk is even increased when such a vapour pipe cooperates with a stirrer located thereunder that can lead to the formation of a vortex in the milk nearby the stirrer and space the pipe's vapour outlet above the surface of the milk which can also lead to the projection of milk in addition to poor or non incorporation of vapour into the milk.
Moreover, the use of interdependent stirrers, vapour and/or air injection means, such as systems using a Venturi arrangement or systems having a stirrer driven by the injected vapour, have a very limited versatility. Normally, they allow only one mode of operation and require the user's skills, e.g. motion of the frothing arrangement and/or of the receptacle to adjust the frothing characteristics.
One solution to this latter problem has been proposed in EP 1 501 398 in which vapour is mixed with air in a ratio determined by the user and then injected as a mixture into a receptacle containing milk. The vapour-air ratio is adjusted with the view to heat the milk with a user-adjusted amount of froth. EP 1 776 905 discloses a similar system in which pulses of compressed air are incorporated into a flow of vapour and then injected as a mixture into a jug of milk. A further system of the same type is disclosed in U.S. Pat. No. 6,006,654 in which milk is pumped into a vapour stream and then mixed with air for frothing and then delivered at a discharge nozzle.
There is therefore still a need to provide a reliable system that is simple and reliable for producing frothed milk.